Leaky-wave antennas are electromagnetic traveling-wave radiators fed at one end and terminated in a resistive load at the other. The feeding end is used to launch a wave that travels along the antenna while leaking energy into free space. Power remaining in the traveling wave is absorbed as it reaches the terminated end. The fact that a single feed is used to excite a leaky-wave antenna results in higher radiation efficiency in comparison with a microstrip antenna array. In addition, a leaky-wave antenna does not suffer from spurious-radiation and ohmic losses associated usually with a corporate-fed microstrip array. The aforementioned features of leaky-wave antennas make them well suited for operation at high frequencies.
In 1979, a traveling-wave microstrip antenna based on the first higher-order mode (EH1) in microstrip was first disclosed. A microstrip is defined herein to be an electromagnetic waveguide made up of conducting traces lying on the top surface of a conductor-backed dielectric slab. The antenna was asymmetrically fed by means of a microstrip line as shown in FIG. 1a, and transverse slots located along the center line of the antenna were used to suppress the fundamental mode. Using a quarter-wave transformer, the input impedance of the antenna was matched to the characteristic impedance of the microstrip feed line. The antenna radiated an x-polarized main beam at an angle θ of 37.5° away from broadside (the z direction). It exhibited an impedance bandwidth broader than that of the resonant microstrip patch, but also produced a high backlobe level.
It was later shown that the microstrip antenna introduced previously could have been operated as a leaky-wave antenna had it been made longer (4.85 times λo long instead of 2.23 times λo, where λo is the free-space wavelength at the design frequency). It was also shown that the high backlobe level exhibited by the previous antenna is due to the fact that 35% of the incident power is reflected at the terminated end, with the backlobe appearing at the same angle as the main beam when measured from broadside. A three-dimensional angled view of the leaky-wave microstrip antenna is shown in FIG. 2.
The main-beam direction of a leaky-wave antenna scans well with frequency. However, attempting to scan the same beam at fixed frequency has so far been either impractical (for example, use of liquid dielectric as disclosed in “Leaky-wave antennas using artificial dielectrics at millimeter-wave frequencies”, Bahl et al., IEEE Transactions on Microwave Theory and Techniques, vol. MTT-28, no. 11, pp.1205–1212, November 1980, or biased ferrite as disclosed in “Experimental studies of magnetically scannable leaky-wave antennas having a corrugated ferrite slab/kielectric layer structure”, Maheri et al., IEEE Transactions on Antennas and Propagation, vol. AP-36, no7, pp. 911–917, July 1988), inefficient (only 50% efficiency at 40 GHz, as disclosed in “Superconductors spur application of ferroelectric films”, Vendik et al., Microwaves & RF, vol. 33, no. 7, pp. 67–70, July 1994), or did not provide a large scan range (only 5°, as disclosed in “Single-frequency electronic-modulated analog-line scanning using a dielectric antenna”, Horn et al., IEEE Transactions on Microwave Theory and Techniques, vol. MTT-30, no. 5, pp. 816–820, May 1982).
In 1998, the leaky-wave microstrip antenna previously disclosed was transformed into a periodic structure as shown in FIGS. 3, 4a and 4b, by Noujeim and Balmain, as discussed in K. M. Noujeim, “Fixed Frequency beam-steerable leaky-wave antennas, “Ph. D. Thesis, University of Toronto, Ontario, Canada, 1998, and K. M. Noujeim and K. G. Balmain, “Fixed-frequency beam-steerable leaky-wave antennas, “XXVIth General Assembly, International Union of Radio Science (URSI), August 1999. Identical varactor diodes were used as phase-shifting elements to series-connect the radiating rectangular patches. Noujeim and Balmain showed that the main beam of the resulting structure may be scanned continuously at fixed frequency by varying the reverse-bias voltage across the varactor diodes from 0 to 900 volts. For a microstrip with a relative dielectric permittivity of 6.15, they obtained a 60° scan range both theoretically and experimentally at a frequency f=5.2 GHz. Due to the fact that the varactor diodes were arranged in series, the maximum voltage required to reverse-bias them is high (900 volts).
Though fixed frequency leaky wave microstrip antennas have developed over the years, there is still a need for better, more efficient implementations. What is needed is a fixed frequency beam-steerable leaky-wave microstrip antenna that improves over the shortcomings and disadvantages over those of the prior art.